The present invention relates to apparatus for dispensing and applying adhesives in general, and more particularly to improvements in apparatus which can be utilized with advantage for the application of adhesive to the backs of signatures or like accumulations of paper sheets or the like in bookbinding machines preparatory to bonding of covers thereto. Still more particularly, the invention relates to improvements in apparatus wherein the commodities, selected portions of which are to be provided with layers of adhesive, are preferably transported by a conveyor having pairs of gripping devices which move along an endless path and serve to advance the commodities along an adhesive applying station. The station normally accommodates a vessel, e.g., a conventional paste tank, wherein the upper level of the supply of confined adhesive is preferably maintained within a predetermined range of levels.
U.S. Pat. No. 4,059,714 discloses an apparatus wherein heated adhesive is foamed prior to the application to selected portions of stacked paper sheets or the like. The apparatus comprises means for admitting an inert gaseous fluid at elevated pressure into a supply of adhesive so that the admitted gas and the adhesive form a foam whose gaseous fraction is permitted to escape during application of the foam to commodities at atmospheric pressure.
German Offenlegungsschrift No. 29 04 097 also discloses an apparatus which serves to apply heated and foamed adhesive to the backs of stacked sheets in a bookbinding machine. The apparatus of this German publication employs a spray nozzle which discharges foamed adhesive against the selected portions of successive stacks. This apparatus exhibits several drawbacks, primarily because it cannot prevent contamination of those portions of commodities which are not to be contacted by adhesive, especially if the thickness of a series of commodities is less than the thickness of the previously treated commodities. In a bookbinding machine, the thickness of paper stacks which are to be provided with covers can fluctuate within a very wide range (e.g., between 3 mm and 80 mm). When the thickness of the next-following stacks is less than that of the preceding stacks, the nozzle is likely to spray adhesive not only against a selected edge face of each stack but also against the sides of the stacks with attendant contamination of the stacks and other problems during further processing. In order to avoid contamination and other inconveniences, the apparatus of the German publication must be furnished with a set of nozzles each of which is designed to discharge a spray of given width. Thus, the operation of the machine must be interrupted for extended intervals of time whenever the machine is to shift from the making of thicker books to the making of thinner books or vice versa with attendant substantial losses in output. Droplets of adhesive which miss their mark (i.e., which fail to land upon the selected edge faces of the commodities) are also likely to contaminate the gripping devices of the transporting means, even if the nozzle is exchanged whenever the machine is to turn out products whose thickness deviates from that of the previously treated products. Deposition of adhesive on the surfaces of the gripping devices interferes with proper disengagement of commodities from such gripping devices downstream of the adhesive applying station and entails further contamination of such commodities.
Another drawback of presently known adhesive applying apparatus which are designed to apply a foamed adhesive is that the foaming or the defoaming operation often takes up excessive intervals of time. Moreover, if a bookbinding machine is to process stacks of paper sheets or the like at a high or very high speed, the thickness of the adhesive layers which are applied to selected edge faces of the stacks is likely to change drastically because the rate of discharge of foamed adhesive per unit of time is constant.
Still further, the cost of the aforediscussed conventional apparatus is rather high because the act of dispersing an inert gas in adhesive can be a rather complex operation. Moreover, the rate of speed at which an inert gas can be caused to leave the liquid fraction of a foamed adhesive is rather low (up to one minute) which renders such types of apparatus useless in many types of modern bookbinding and other machines wherein the commodities must be processed at the rate of up to and in excess of thirty per minute. Thus, if the foamed adhesive contains an inert gas which was admitted into liquid adhesive at an elevated pressure, it takes up to one minute to ensure adequate segregation of gaseous fraction from the liquid fraction if such segregation is to take place at atmospheric pressure after the foam issues from the orifice of a spray nozzle.